The invention relates to a method of supplying air to a recovery boiler, in which method the air needed for combustion is supplied to the recovery boiler at various levels of the recovery boiler in the vertical direction, at at least one air supply level the air being supplied to the recovery boiler in such a way that a vortex spiralling around the vertical axis is formed in the recovery boiler.
The invention also relates to an arrangement for supplying air to a recovery boiler, the arrangement comprising air nozzles at various levels of the recovery boiler in the vertical direction, the nozzles at at least one air supply level being arranged to supply air to the recovery boiler in such a way that a vortex spiralling around the vertical axis is formed in the recovery boiler.
In recovery boilers, various ways of supplying air are used, so that black liquor would burn as efficiently as possible and yet the combustion process could be controlled in a desired manner in both the horizontal and the vertical directions of the boiler. Typically, air is supplied at various levels in the vertical direction of the recovery boiler so as to cause sub-stoichiometric combustion in the gas flow direction as far as possible, i.e. in the vertical direction of the recovery boiler. The final air causing stoichiometric combustion is not fed until the final, typically tertiary step. Solutions like this are known, for example, from U.S. Pat. No. 5,007,354.
A problem in the above solution is that to make the combustion efficient, the droplets of fuel should be as small as possible so that the fuel and the combustion air would mix as thoroughly as possible. As a result of this, however, the particulate fuel droplets tend to move with the gas flow to the upper parts of the furnace before burning, which defers the combustion step too much, and so the combustion is no longer efficient and the emissions are not reduced efficiently. With regard to the emissions, it would be advantageous if the combustion were sub-stoichiometric as far as possible, so that essentially no No.sub.x compounds would be formed. As the thermal value is also low, the combustion is not so efficient. Also, the fact that the droplets move up with the gas flow and do not burn until after this may make the temperature close to the superheaters rise too high, which speeds the corrosion of the superheaters and thereby shortens their effective life.
A solution suggested to the problem in Finnish Patent Application No. 931,123 is that the nozzles are not placed in horizontal supply layers but in a plural number of arrays of nozzles on top of one another so as to make the air supply more efficient with respect to burning. The solution, however, does not solve the problem in essence. The structure presented in the application is difficult to build, and the variations in the air distribution in the vertical direction that are required by the combustion process are difficult to accomplish.
In all the solutions, problems are posed by the channelling of the flows in the upper part of the furnace and by different vertical backflows, whereby the volume of the furnace is not actually used efficiently with respect to the reactions, and so the walls cannot be used efficiently for heat transfer.
U.S. Pat. No. 5,450,803 teaches a solution in which secondary air is supplied to a recovery boiler before a black liquor supply point so as to make the secondary air spin. This forms a vertical vortex in the recovery boiler. A problem in the solution is that by the effect of the centrifugal force generated by the vortex, droplets of black liquor assemble on the walls of the furnace, blocking, for example, nozzle apertures. It has also been noted that as a result of this, a hole tends to form in the middle of the bed of the recovery boiler, which increases the stress that the bottom of the recovery boiler is subjected to. Further, as the spinning motion of the flue gases caused by the vortex tends to last, this also causes distortion of the flow at the superheaters, which both weakens the operation of the superheaters and causes exceptional accumulation of deposit in them.
In the lecture "The Chemical Recovery Boiler Optimized Air System" by Lefebvre Burell, given in TAPPI Kraft Recovery Operations Seminar in Orlando on 10th to 15th of Jan. 1988, a solution was proposed in which tertiary air was supplied by making the air jets cross so that a vortex was formed in the middle of the recovery boiler. In this solution, the problem is that in addition to the vortex desired, separate uncontrolled local vortices were formed, and these made further droplets accumulate on the walls of the recovery boiler. Further, spinning performed at the tertiary level did not bring about the expected advantages in the action and combustion of the black liquor droplets: for example, the advantages brought about by quicker drying of the black liquor droplets were not achieved.